April 29, 1930
Airmail Act of 1930
The Airmail Act of 1930, also known as the McNary Watres Act, was passed on April 29, 1930, and gave Postmaster General Brown authority to change postal policy. It also gave him the authority to make those changes based on his own personal preferences. When Brown was done, all but three US airlines were out of business.
http://avstop.com/History/NeedRegulations/Act1930.htm